Application of construction waste residue-based compositing fillers in bioretention facility: Intensified nitrogen removal and mitigated by-product effects.

Bioretention facilities (BRF) are widely utilized in sponge cities for stormwater management, but they face challenges in effectively controlling nitrogen due to the instability of traditional fillers. Pyrite has been extensively employed as an electron donor to enhance nitrogen removal; however, it generates by-products such as H+, SO42-, and Fe2+/Fe3+ that can pose environmental issues. Construction waste residues (CWR), consisting of waste brick and concrete, have been explored as alternative fillers, but they suffer from alkaline leaching problems. To mitigate this limitation, this study investigated the optimal packing location of a composite filler consisting of pyrite with CWR (a mixture of waste brick and concrete) within BRF, considering rainfall intensity and antecedent drying days. The synergetic pollution removal and by-product mitigate mechanism of waste residue-based composite fillers applied to BRF was elucidated through SEM and FTIR characterizations, as well as microbial community analysis. Results showed that incorporating 40% CWR in the vadose layer and 20% pyrite in the submerged layer significantly enhanced nitrogen removal while maintaining limited NO2--N accumulation in BRF (exceeding 80% TN), effectively controlling effluent pH levels and by-products (total Fe and SO42-) concentration within acceptable limits. CWR improved NH4+-N and TP adsorption capacity, whereas pyrite further increased NO3--N removal through autotrophic denitrification. Alkaline leaching from CWR was neutralized with H+ produced during autotrophic denitrification, forming Fe (oxyhydr)oxides with Fe3+ derived from pyrite oxidation; thus, achieving acceptable pH values and total Fe concentrations. Meanwhile, the media composite strategy successfully regulated effluent SO42- concentration by reducing Thauera and Thiobacillus abundance. Overall, this study demonstrates that integrating CWR with pyrite into BRF can facilitate stable and efficient pollutant removal while concurrently mitigating by-product issues.